Is it possible to add an external user to an issue already created in the web UI?

For example, I've created an issue, but I want someone to be able to add comments and join the discussion. They are external to the company, so not a regular user.

Whereas normally, the user emails "support+whatever" and a ticket is created. The ticket already exists and now "[email protected]" needs to see the convo. There are no current emails, as it's just me talking to myself on these tickets.

Alongside that, can others be added to tickets (i.e. CC'd or Forwarded the email chain)?